1. Field of the Invention
The present invention relates generally to the field of portable telephones. More particularly, it relates to a portable telephone with improvements for directing the radiated power away from the telephone user""s head.
2. Prior Art
Cellular telephones and cordless telephones (collectively referred to as xe2x80x9cportable telephonesxe2x80x9d) are widely used. In recent years, concerns have been expressed about the health effects of long term exposure to the energy radiated by portable telephones. A cellular telephone or cordless telephone uses frequencies in the VHF to microwave range to reach a base station, which then relays communications through a telecommunications network to a destination. Each telephone relies on a transmission antenna that radiates energy outwardly in all directions. Because this antenna is held very close to the user""s head, a significant portion of the radiated energy passes through the head on its way to the base station. One estimate places the power absorbed by the head at as high as 50% of the total radiated power. Since an average cellular telephone radiates a power of 600 milliwatts, the extended use of a telephone, for example over 20 minutes, will mean an energy deposit greater than 720 joules into the head. This amount of energy is sufficient to raise the temperature of 17 gm of water by 10 degrees.
In view of the fact that the radiating frequency of a cellular telephone is as high as 1.8 GHz and that of a cordless telephone has reached the high frequency of 2.4 GHz, which is very close to the characteristic frequency of water molecules commonly used in microwave ovens (2.45 GHz), it is important to shield the user from such potentially harmful radiation. Even though precise medical studies on the effect of radiation upon the human brain are not yet complete, prudence compels finding a method of protecting the portable telephone user from unnecessary and potentially harmful radiation. With the anticipated rapidly expanding market for cellular and cordless phones, even higher frequencies will certainly be used to meet the demand for increased bandwidth. The need to protect the telephone users from harmful radiation will be correspondingly greater.
Furthermore, since it is desirable to make portable telephones as light and compact as possible, energy efficiency is an important design consideration. By avoiding the undesirable propagation and absorption of radiation into the user""s head, a portable telephone can more efficiently direct radiation into a wide angle away from the user""s head.
Various shielding devices have been proposed for portable telephones. For example, such devices are shown in U.S. Pat. Nos. 5,367,309; 5,373,304; 5,444,866; 5,525,988; 5,541,609; 5,507,012; 5,550,552; 5,564,085; 5,613,221; 5,657,386; 5,666,125; 5,694,137; 5,726,383; 5,777,586; 5,787,340; and 5,826,201.
Another approach, and one that is more relevant to the present invention, is disclosed in U.S. Pat. No. 5,530,919. This patent discloses a mobile communicator, such as a cellular telephone, in which the energy radiated by the transmission antenna in the direction of the user""s head is attenuated by a ground electrode pattern on an internal circuit board. As disclosed, the transmission antenna comprises a miniature surface-mounted antenna on a circuit board. The opposite side of the circuit board has a ground electrode pattern covering an area that extends beyond the outline of the antenna.
The present invention provides an improved method of shielding the user of a portable telephone from the energy radiated by the transmission antenna. In one embodiment, an external transmission antenna is disposed within a jacket for the portable telephone. A grounded metallic surface is interposed between the antenna and the user""s head. The metallic surface is spaced apart from the radiating element of the transmission antenna by one-quarter of the effective wavelength, so as to maximize the reflection of energy away from the user""s head.